1. Technical Field
The present invention relates to a detection device for performing quantitative analysis on an infinitesimal substance on which qualitative analysis is to be performed.
2. Related Art
In recent years, surface enhanced Raman scattering (SERS) spectroscopy using surface plasmon resonance (SPR), and in particular, localized surface plasmon resonance (LSPR), has been attracting attention as a sensitive spectroscopic technique for detecting low-density sample molecules (e.g., see Japanese Patent No. 3714671, JP-A-2000-356587). Surface enhanced Raman scattering denotes a phenomenon that the Raman scattering light is enhanced 102 through 1014 times on a metal surface having nanometric unevenness. In surface enhanced Raman scattering, the sample molecules are irradiated with an excitation light having a single wavelength such as a laser. The light with a scattering wavelength (the Raman scattering light) shifted from the wavelength of the excitation light by as much as the molecular vibration energy of the sample molecules is spectroscopically detected to thereby obtain the fingerprint spectrum of the sample molecules. As a result, it becomes possible to identify the sample molecules based on the shape of the fingerprint spectrum.
Although surface enhanced Raman scattering can be used for qualitative detection in an infinitesimal concentration of gas molecules due to the electric field enhancement effect of the localized surface plasmon resonance, quantitative analysis has not yet been achieved. One of the causes of this is that the enhanced electric field intensity generated in the localized surface plasmon resonance is exponentially attenuated from the maximum enhanced electric field. According to “Design and Applied Technology of Plasmon Nanomaterial” by CMC Publishing Co., Ltd., p. 181, the SERS intensity “I” caused by the enhanced electric field has been experimentally calculated to have the following relationship with the distance “r” from the enhanced electric field surface.
                    I        =                              (                          1              +                              r                a                                      )                                -            10                                              (        1        )            
Here, “a” denotes the radius of a metal nanoparticle. Formula 1 suggests that the SERS intensity varies irrespective of the number of molecules. When the number of surface-adsorbed molecules is large (the coverage is high), it is possible to perform quantitative evaluation based on an ensemble-averaged signal. However, when the coverage is low, an individual molecule provides the intensity expressed by Formula 1, and therefore, the quantitative evaluation is difficult.
JP-A-2009-103651 proposes a SERS quantitative analysis technique wherein molecule samples with a known SERS spectrum are fixed inside a substrate, and quantitative analysis is performed by comparison with the spectrum intensity thereof. However, if the surface coverage of the detection target molecules is low, the number of molecules adsorbed to the enhanced spot decreases, and the effect of Formula 1 becomes conspicuous, and the SERS intensity varies dramatically. Therefore, the proposed method fails to work when the adsorption coverage of the detection target molecules is low (e.g., with an extremely low concentration sample, or a short exposure duration).
JP-T-2008-513772 proposes a quantitative detection device having a combination of attenuated total reflection surface plasmon resonance (SPR) and a quartz crystal microbalance (QCM). Although JP-T-2008-513772 states in paragraphs 0003 and 0005 that the quantitative analysis accuracies of the SPR and the QCM are about 1 ng/cm2, and further states in paragraph 0102 that the two quantitative analysis signals are complementary to each other, there is no specific explanation regarding how the quantitative analysis is performed based on the two signals and, in particular, how the quantitative analysis is performed on an infinitesimal substance. Moreover, since neither the SPR nor the QCM can measure the fingerprint spectrum of the sample molecules of which the quantity is to be determined, it is also technically unachievable to qualitatively detect the sample molecules.